Effects of chronic caffeine on patterns of brain blood flow and behavior throughout the sleep-wake cycle in freely behaving mice.

Caffeine has significant effects on neurovascular activity and behavior throughout the sleep-wake cycle. We used a minimally invasive microchip/video system to continuously record effects of caffeine in the drinking water of freely behaving mice. Chronic caffeine shifted both rest and active phases by up to 2 h relative to the light-dark cycle in a dose-dependent fashion. There was a particular delay in the onset of rapid eye movement (REM) sleep as compared with non-REM sleep during the rest phase. Chronic caffeine increased wakefulness during the active phase and consolidated sleep during the rest phase; overall, there was no net change in the amount of time spent in the wake, sleep, or REM sleep states during caffeine administration. Despite these effects on wakefulness and sleep, chronic caffeine decreased mean cerebral blood volume (CBV) during the active phase and increased mean CBV during the rest phase. Chronic caffeine also increased heart rate variability in both the sleep and wake states. These results provide new insight into the effects of caffeine on the biology of the sleep-wake cycle. Increased blood flow during sleep caused by chronic caffeine may have implications for its potential neuroprotective effects through vascular mechanisms of brain waste clearance.

Different characteristics of cortical spreading depression in the sleep and wake states.

OBJECTIVE: The objective of this study is to characterize the effects of the sleep-wake cycle on neurovascular and behavioral characteristics of cortical spreading depression (CSD). BACKGROUND: There is an important bi-directional relationship between migraine and the sleep-wake cycle, but the basic mechanisms of this relationship are poorly understood. METHODS: We have developed a minimally invasive microchip system to continuously monitor cerebral blood volume (CBV) with optical intrinsic signal (OIS), head movement, and multiple other physiological and behavioral parameters in freely behaving mice over weeks. Behavior is also monitored with simultaneous video recording. This system can also be used to intermittently trigger and record CSD and accompanying neurovascular and behavioral responses. CSD was triggered optically in different stages of the sleep-wake cycle. RESULTS: The optical stimulus threshold to trigger CSD was significantly higher in the wake state compared to sleep (stimulation duration = 16.4 ± 9.7 s vs. 10.8 ± 5.8 s, p = 0.037, n = 6 mice). CSD evoked in the wake versus sleep state produced changes in CBV that were smaller (largest relative change -4.5 ± 5.0% ∆OIS vs. -14.3 ± 8.5% ∆OIS, p = 0.001) and shorter in duration (33:22 ± 6:37 vs. 49:42 ± 8:05 min:s, p = 0.012, n = 6 mice). The threshold for CSD and kinetics of associated CBV changes were correlated with the time since falling asleep or awakening (n = 47 CSDs in 6 mice). CSD triggered in the wake state was associated with a transient freezing behavior. CSD triggered during sleep typically caused a transient awakening and behavioral response. This was followed by a return to sleep until recovery from the sustained phase of decreased CBV that occurred 30-60 min later, at which time there was consistent awakening with behaviors similar to those that occurred at CSD onset. CSD triggered in the wake state evoked a transient decrease in heart rate (from 11.9 ± 0.8 to 9.6 ± 0.8 Hz, p = 0.002, n = 5), whereas when triggered in the sleep state there was a transient increase in HR (from 7.5 ± 0.4 Hz to 9.3 ± 1.1 Hz, p = 0.016, n = 5). CONCLUSIONS: The sleep-wake cycle has significant effects on CSD that may have relevance to the clinical presentations of migraine and brain injury.

Continuous long-term recording and triggering of brain neurovascular activity and behaviour in freely moving rodents.

A minimally invasive, microchip-based approach enables continuous long-term recording of brain neurovascular activity, heart rate, and head movement in freely behaving rodents. This approach can also be used for transcranial optical triggering of cortical activity in mice expressing channelrhodopsin. The system uses optical intrinsic signal recording to measure cerebral blood volume, which under baseline conditions is correlated with spontaneous neuronal activity. The arterial pulse and breathing can be quantified as a component of the optical intrinsic signal. Multi-directional head movement is measured simultaneously with a movement sensor. A separate movement tracking element through a camera enables precise mapping of overall movement within an enclosure. Data is processed by a dedicated single board computer, and streamed from multiple enclosures to a central server, enabling simultaneous remote monitoring and triggering in many subjects. One application of this system described here is the characterization of changes in of cerebral blood volume, heart rate and behaviour that occur with the sleep-wake cycle over weeks. Another application is optical triggering and recording of cortical spreading depression (CSD), the slowly propagated wave of neurovascular activity that occurs in the setting of brain injury and migraine aura. The neurovascular features of CSD are remarkably different in the awake vs. anaesthetized state in the same mouse. With its capacity to continuously and synchronously record multiple types of physiological and behavioural data over extended time periods in combination with intermittent triggering of brain activity, this inexpensive method has the potential for widespread practical application in rodent research. KEY POINTS: Recording and triggering of brain activity in mice and rats has typically required breaching the skull, and experiments are often performed under anaesthesia A minimally invasive microchip system enables continuous recording and triggering of neurovascular activity, and analysis of heart rate and behaviour in freely behaving rodents over weeks This system can be used to characterize physiological and behavioural changes associated with the sleep-wake cycle over extended time periods This approach can also be used with mice expressing channelrhodopsin to trigger and record cortical spreading depression (CSD) in freely behaving subjects. The neurovascular responses to CSD are remarkably different under anaesthesia compared with the awake state. The method is inexpensive and straightforward to employ at a relatively large scale. It enables translational investigation of a wide range of physiological and pathological conditions in rodent models of neurological and systemic diseases.